U.S. patent application number 10/662097 was filed with the patent office on 2004-03-18 for fiber amplifier system for producing visible light.
This patent application is currently assigned to Lightwave Electronics Corporation. Invention is credited to Kane, Thomas J., Keaton, Gregory L., Morehead, James J..
Application Number | 20040052278 10/662097 |
Document ID | / |
Family ID | 46299955 |
Filed Date | 2004-03-18 |
United States Patent
Application |
20040052278 |
Kind Code |
A1 |
Kane, Thomas J. ; et
al. |
March 18, 2004 |
Fiber amplifier system for producing visible light
Abstract
A light source is disclosed having a pulsed laser, a fiber
amplifier optically coupled to the pulsed laser, and a nonlinear
frequency converting element optically coupled to the fiber
amplifier. The pulsed laser, e.g., a passively Q-switched laser, is
configured to generate light pulses characterized by a pulse length
of less than about 1.7 nsec and sufficiently large that a frequency
bandwidth of the pulses after they emerge from the fiber amplifier
is less than an acceptance bandwidth of the nonlinear frequency
converting element. The laser is pulsed at a pulse repetition rate
sufficiently large that the fiber amplifier does not spontaneously
emit radiation between pulses. In such a source, the fiber
amplifier is substantially free of stimulated Brillouin scattering
and self-phase modulation may be held to a level that does not
reduce conversion of infrared radiation to visible radiation. Such
a light source can be combined with an image generator and a
scanner in an image projection system.
Inventors: |
Kane, Thomas J.; (Menlo
Park, CA) ; Keaton, Gregory L.; (San Francisco,
CA) ; Morehead, James J.; (Oakland, CA) |
Correspondence
Address: |
JOSHUA D. ISENBERG
204 CASTRO LANE
FREMONT
CA
94539
US
|
Assignee: |
Lightwave Electronics
Corporation
Mountain View
CA
|
Family ID: |
46299955 |
Appl. No.: |
10/662097 |
Filed: |
September 12, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10662097 |
Sep 12, 2003 |
|
|
|
09927145 |
Aug 10, 2001 |
|
|
|
Current U.S.
Class: |
372/25 ;
359/341.1; 372/10 |
Current CPC
Class: |
H01S 3/06729 20130101;
H01S 3/113 20130101; H01S 3/06754 20130101; H01S 3/108
20130101 |
Class at
Publication: |
372/025 ;
359/341.1; 372/010 |
International
Class: |
H01S 003/11; H01S
003/10; H01S 003/00; H04B 010/12 |
Goverment Interests
[0002] This invention was made under contract # F29601-01-C-0246 of
the United States Air Force. The government has certain rights in
this invention.
Claims
What is claimed is:
1. A fiber amplifier system comprising: a pulsed laser configured
to generate light pulses characterized by a pulse length
T.sub.pulse and a repetition rate; a fiber amplifier optically
coupled to the pulsed laser; and a nonlinear frequency converting
element optically coupled to the fiber amplifier, wherein the pulse
length T.sub.pulse is less than about 1.7 nsec and sufficiently
large that a frequency bandwidth of the pulses after they emerge
from the fiber amplifier is less than an acceptance bandwidth of
the nonlinear frequency converting element; wherein the repetition
rate is sufficiently large that amplified spontaneous emission in
the fiber amplifier between pulses does not extract more than 50%
of the total power from the fiber amplifier.
2. The fiber amplifier system of claim 1 wherein the repetition
rate is greater than about 100 kHz.
3. The fiber amplifier system of claim 2 wherein the pulse length
T.sub.pulse is greater than about 100 psec.
4. The fiber amplifier system of claim 2 wherein the pulsed laser
is a passively Q-switched laser (PQSL).
5. The fiber amplifier system of claim 4, further comprising a PQSL
pump source optically coupled to the PQSL.
6. The fiber amplifier system of claim 1, further comprising a
fiber pump source optically coupled to the fiber amplifier.
7. The fiber amplifier system of claim 1, wherein the fiber
amplifier is characterized by a figure of merit z that is greater
than about 0.1, wherein z is given by z=(0.037) .beta.(dB/m)
A.sub.mode(.mu.m.sup.2), where .beta. (dB/m) is the absorption of
the fiber amplifier in dB/meter and A.sub.mode is the mode area of
light to be amplified by the fiber amplifier.
8. The fiber amplifier system of claim 7 wherein the figure of
merit z is greater than about 0.5.
9. The fiber amplifier system of claim 7 wherein the fiber
amplifier uses a cladding-pumped fiber with an air cladding.
10. The fiber amplifier system of claim 7 wherein the fiber
amplifier includes a core of refractive index n.sub.c, a depressed
cladding of refractive index n' and an outer cladding of refractive
index n.sub.oc, wherein n'<n.sub.oc<n.sub.c.
11. The fiber amplifier system of claim 7 wherein the fiber
amplifier has a core with an elliptical cross-section.
12. The fiber amplifier system of claim 7 wherein the fiber
amplifier has a W-shaped refractive index profile characterized by
a core with a refractive index n.sub.core and a radius r.sub.c, a
tunnel cladding surrounding the core, the tunnel cladding having a
refractive index n' and a cladding region surrounding the tunnel
cladding, the cladding region having a refractive index n.sub.cl,
wherein n'<n.sub.cl<n.sub.core wherein the core is a
single-mode core characterized by a cutoff V-number V.sub.cl
greater than about 3.0, where 15 V c1 = 2 r c c1 n core 2 - n c1 2
,and where .lambda..sub.cl is a cutoff wavelength for a second mode
of the core.
13. The fiber amplifier system of claim 1 wherein the fiber
amplifier amplifies a primary signal having a wavelength ranging
from about 860 nm to about 1100 nm.
14. The fiber amplifier system of claim 13 wherein the nonlinear
element converts the primary signal to an output signal having a
wavelength ranging from about 430 nm to about 550 nm.
15. An image projection system, comprising: a pulsed laser
configured to generate light pulses characterized by a pulse length
T.sub.pulse and a repetition rate; a fiber amplifier optically
coupled to the pulsed laser; a nonlinear frequency converting
element optically coupled to the fiber amplifier; an image
generator optically coupled to the nonlinear frequency converting
element; and a scanner optically coupled to the image generator,
wherein the pulse length T.sub.pulse is less than about 1.7 nsec
and sufficiently large that a frequency bandwidth of the pulses
after they emerge from the fiber amplifier is less than an
acceptance bandwidth of the nonlinear frequency converting element;
wherein the repetition rate is sufficiently large that amplified
spontaneous emission in the fiber amplifier between pulses does not
extract more than 50% of the total power from the fiber
amplifier.
16. The image projection system of claim 15 wherein the pulsed
laser is configured to generate light pulses at a repetition rate
of greater than about 100 kHz.
17. The image projection system of claim 16 wherein the pulse
length T.sub.pulse is greater than about 100 psec.
18. The image projection system of claim 16 wherein the pulsed
laser is a passively Q-switched laser (PQSL).
19. The image projection system of claim 18 further comprising a
PQSL pump source optically coupled to the PQSL.
20. The image projection system of claim 15 further comprising a
fiber pump source optically coupled to the fiber amplifier.
21. The image projection system of claim 15 wherein the fiber
amplifier is characterized by a figure of merit z that is greater
than about 0.1, wherein z is given by z=(0.037) .beta.(dB/m)
A.sub.mode(.mu.m.sup.2), where .beta.(dB/m) is the absorption of
the fiber amplifier in dB/meter and A.sub.mode is the mode area of
light to be amplified by the fiber amplifier.
22. The image projection system of claim 21 wherein the figure of
merit z is greater than about 0.5.
23. The image projection system of claim 21 wherein the fiber
amplifier uses a cladding-pumped fiber with an air cladding.
24. The image projection system of claim 21 wherein the fiber
amplifier includes a core of refractive index n.sub.c, a depressed
cladding of refractive index n' and an outer cladding of refractive
index n.sub.oc, wherein n'<n.sub.oc<n.sub.c.
25. The image projection system of claim 21 wherein the fiber
amplifier has a core with an elliptical cross-section.
26. The image projection system of claim 15 wherein the fiber
amplifier amplifies a primary signal having a wavelength ranging
from about 860 nm to about 1100 nm.
27. The image projection system of claim 26 wherein the nonlinear
element converts the primary signal to an output signal having a
wavelength ranging from about 430 nm to about 550 nm.
28. A light source comprising: means for generating light pulses
characterized by a pulse length T.sub.pulse and a repetition rate;
means for amplifying the light pulses; and nonlinear means for
frequency converting light pulses that have been amplified by the
amplifying means, wherein the pulse length T.sub.pulse is less than
about 1.7 nsec and sufficiently large that a frequency bandwidth of
the pulses after they emerge from the fiber amplifier is less than
an acceptance bandwidth of the nonlinear frequency converting
element; wherein the repetition rate is sufficiently large that
amplified spontaneous emission in the fiber amplifier between
pulses does not extract more than 50% of the total power from the
fiber amplifier.
29. For an apparatus having a fiber amplifier optically coupled to
the pulsed laser; and a nonlinear frequency converting element
optically coupled to the fiber amplifier, a method for optimizing
the fiber amplifier, the method comprising: determining a
conversion efficiency .delta.(p) of the nonlinear frequency
converting element as a function of a peak power of an input signal
coupled into the fiber amplifier; calculating an average power of
output radiation B(z, p) from the nonlinear frequency converting
element as a function of the peak power p and a figure of merit z,
where z=(0.037).beta.A.sub.mode, where .beta. is a rate of
absorption of pump radiation by the fiber amplifier in dB/m, and
A.sub.mode is a mode area of radiation to be amplified by the fiber
amplifier in um.sup.2, and where 16 B ( z , p ) = ( p ) P ( 1 - - z
p ) , where .epsilon. is a conversion efficiency of the fiber
amplifier, P is an average power of a pump radiation coupled into
the fiber amplifier; determining one or more best values p.sub.0 of
the peak power p for one or more corresponding values of z by
solving 17 B ( z , p ) p | p 0 = 0 ; substituting the best values
p.sub.0 into B(z, p) to determine one or more best values
B.sub.best(z) of the average power of the output radiation from the
nonlinear frequency converting element as a function of the figure
of merit z determining a desired value B.sub.d of the average power
of output radiation from the nonlinear frequency converting element
from requirements of an application for which the apparatus is to
be used; from B.sub.d and the one or more values of B.sub.best(z)
determining a minimum value z.sub.min of the figure of merit for
the fiber; and from z.sub.min selecting a fiber amplifier
characterized by values of .beta. and A.sub.mode such that for the
fiber amplifier z is greater than or equal to z.sub.min.
Description
CROSS-REFERENCE TO A RELATED APPLICATION
[0001] This application is a continuation in part of commonly
assigned co-pending U.S. patent application Ser. No. 09/927,145 to
Thomas Kane and Mark Arbore, entitled "COMPOUND LIGHT SOURCE
EMPLOYING PASSIVE Q-SWITCHING AND NONLINEAR FREQUENCY CONVERSION,
filed Aug. 10, 2001," the disclosures of which are incorporated
herein by reference. This application is also related to
commonly-assigned U.S. patent application Ser. No. ______ (Agent's
Docket Number LEL-0 10) to Thomas J. Kane entitled "HIGH REPETITION
RATE PASSIVELY Q-SWITCHED LASER FOR BLUE LASER BASED ON
INTERACTIONS IN FIBER," which is filed concurrently herewith and
the disclosures of which are incorporated herein by reference.
FIELD OF THE INVENTION
[0003] The present invention relates generally to compound light
sources employing lasers with passive Q-switches and nonlinear
frequency converters to generate light in a desired wavelength
range.
BACKGROUND OF THE INVENTION
[0004] Many applications require reliable, stable and efficient
spectrally-pure high-power light sources. For example, image
projection systems require light sources which exhibit these
characteristics and deliver in excess of 1 Watt average power.
These light sources should be inexpensive to produce and they need
to generate output frequencies in the blue range and in the green
range. For other applications light in the UV range is
required.
[0005] The prior art teaches various types of light sources for
generating light in the visible and UV ranges, including
frequencies corresponding to blue and green light. A number of
these sources rely on a nonlinear frequency conversion operation
such as second harmonic generation (SHG) to transform a frequency
outside the visible range, e.g., in the IR range, to the desired
visible or UV frequency. For example, U.S. Pat. No. 5,751,751 to
Hargis et al. teaches the use of SHG to produce deep blue light.
Specifically, Hargis et al. use a microlaser which has a rare earth
doped microlaser crystal and emits light at about 914 nm to drive
SHG in a crystal of BBO producing output at about 457 nm.
[0006] U.S. Pat. No. 5,483,546 to Johnson et al. teaches a sensing
system for high sensitivity spectroscopic measurements. This system
uses a passively Q-switched laser emitting light at a first
frequency. The light from the laser is transmitted through a fiber
and converted to output light at a second frequency in the UV
range. The conversion is performed by two frequency doubling
crystals disposed far away from the Q-switched laser.
[0007] U.S. Pat. No. 6,185,236 to Eichenholz et al. teaches a self
frequency doubled Nd:doped YCOB laser. The laser generates light of
about 400 mW power at about 1060 nm and frequency doubles it with
the aid of a frequency doubling oxyborate crystal to output light
in the green range at about 530 nm. Eichenholz et al. combine the
active gain medium and the frequency doubler in one single element
to produce a compact and efficient light source.
[0008] In U.S. Pat. No. 5,909,306 Goldberg et al. teach a
solid-state spectrally pure pulsed fiber amplifier laser system for
generating UV light. This system has a fiber amplifier in a
resonant cavity and an acousto-optic or electro-optic modulator
incorporated into the cavity for extracting high-peak-power,
short-duration pulses from the cavity. These short pulses are then
frequency converted in several non-linear frequency conversion
crystals (frequency doubling crystals). The addition of the
modulator into the cavity for extracting the pulses and placement
of the fiber amplifier within the resonant cavity renders this
system very stable and capable of delivering a spectrally-pure
pulse. Unfortunately, this also makes the system too cumbersome and
expensive for many practical applications such as display
systems.
[0009] U.S. Pat. No. 5,740,190 to Moulton teaches a three-color
coherent light system adapted for image display purposes. This
system employs a laser source and a frequency doubling crystal to
generate green light at 523.5 nm. Moulton's system also generates
blue light at 455 nm and red light at 618 nm by relying on
frequency doubling and the nonlinear process of optical parametric
oscillation.
[0010] Unfortunately, the light sources described above and various
other types of light sources taught by the prior art can not be
employed to make stable, low-cost, efficient sources of light
delivering 1 Watt of average power for display applications. This
is in part due to the fact that frequency conversion, e.g.,
frequency doubling in crystals, is not a very efficient operation.
If the frequency doubling crystal had extremely high non-linearity,
then low power continuous wave (cw) lasers could be efficiently
doubled to generate output power levels near 1 Watt. However, in
the absence of such frequency doubling crystals high-peak-power,
short pulse lasers have to be used to obtain frequency doubled
light at appreciable power levels. It should also be noted that
providing such high-peak-power short pulses adds complexity to the
design of the light sources and introduces additional costs.
[0011] U.S. Pat. No. 5,394,413 to Zayhowski addresses the issue of
efficient frequency doubling by using a passively Q-switched
picosecond microlaser to deliver the pulses of light. Such pulses
can be efficiently converted, as further taught by Zayhowski in a
frequency-doubling crystal. Devices built according to Zayhowski's
teaching operate at relatively low average power levels and low
repetition rates. Attempts to increase these parameters by pumping
the microchip harder will cause multiple transverse-mode operation
leading to degradation of beam quality and also incur increased
pulse-to-pulse noise. Hence, Zayhowski's devices can not be used in
applications such as projection displays, which require high
average power and high repetition rates and good beam quality.
[0012] Hence, what is needed is a stable and efficient source of
light in the blue and green ranges which can be used in a
projection display.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The teachings of the present invention can be readily
understood by considering the following detailed description in
conjunction with the accompanying drawings, in which:
[0014] FIG. 1 is a diagram of a light source according to the
invention.
[0015] FIG. 2 is a timing diagram illustrating pulse timing in the
light source of FIG. 1.
[0016] FIG. 3A is a detailed cross sectional view of a particular
Q-switched laser suitable for use in a light source according to
the invention.
[0017] FIG. 3B is a diagram of another Q-switched laser suitable
for use in a light source according to the invention.
[0018] FIGS. 4A&B are cross sectional views of fiber amplifiers
suitable for use in a light source of the invention.
[0019] FIG. 5 is a diagram of another embodiment of a light source
according to the invention.
[0020] FIG. 6 is an isometric view of a display system in
accordance with the invention.
[0021] FIG. 7 is a plan view of a pixel in the display system of
FIG. 6.
[0022] FIG. 8 is a timing diagram illustrating the synchronization
of the refresh rate with the pulse rate.
[0023] FIG. 9 depicts a graph of Brillouin threshold versus pulse
length.
[0024] FIG. 10 depicts a graph of a best value of the average power
of the output radiation from a fiber amplifier as a function of a
figure of merit for the fiber amplifier according to an embodiment
of the present invention.
[0025] FIG. 11A depicts a refractive index profile for a
conventional fiber.
[0026] FIGS. 11B-11C depict refractive index profiles for rejecting
undesired wavelengths from the core of a fiber according to an
embodiment of the present invention.
[0027] FIG. 12 depicts a cross-sectional schematic diagram of an
air-clad fiber of a type that may be used with embodiments of the
present invention.
DESCRIPTION OF THE SPECIFIC EMBODIMENTS
[0028] Although the following detailed description contains many
specific details for the purposes of illustration, anyone of
ordinary skill in the art will appreciate that many variations and
alterations to the following details are within the scope of the
invention. In the mathematical derivations described below certain
assumptions have been made for the sake of clarity. These
assumptions should not be construed as limitations on the
invention. Accordingly, the embodiments of the invention described
below are set forth without any loss of generality to, and without
imposing limitations upon, the claimed invention.
[0029] In embodiments of the present invention, a pulsed laser,
preferably a passively Q-switched laser (PQSL) launches a signal
into a fiber amplifier. The pulsed laser generates pulses of width
less than about 1.7 nsec. The pulse length can be greater than
about 100 psec and the repetition rate can be greater than about
100 kHz. The fiber amplifier has a pump guide for receiving pump
light (preferably from diode lasers), and embedded within this pump
guide is a core. In some, but not all, embodiments of the
invention, the core includes a depressed cladding. The light from
the Q-switched laser is coupled to the core of the fiber, and is
amplified by the fiber. The amplified light then leaves the fiber
and enters a nonlinear frequency-converting element, preferably
comprising one or more nonlinear crystals such as lithium borate
(LBO). The resulting frequency-converted light is the desired,
generally visible light. Embodiments of the present invention are
free of stimulated Brillouin scattering (SBS) and have levels of
self-phase modulation that does not reduce frequency conversion to
the visible.
[0030] I. Fiber Amplifier System
[0031] FIG. 1 illustrates a fiber amplifier system 10 according to
an embodiment of the invention. The fiber amplifier system 10 has a
passively Q-switched laser (PQSL) 12, a fiber amplifier 14 and a
nonlinear frequency converting element 60. The PQSL 12 generates a
primary beam 34 of primary pulses 36. The primary pulses have a
pulse length less than about 1.7 nsec and sufficiently large to
facilitate frequency conversion in the nonlinear element 60, e.g.,
greater than about 100 psec. The PQSL 12 preferably produces the
pulses at a sufficiently large repetition rate, e.g., greater than
about 100 kHz. The fiber amplifier system 10 can also have a PQSL
pump source 16 for producing PQSL pump light 20 that pumps the PQSL
12. In this embodiment, the PQSL pump source 16 is a laser equipped
with a wavelength tuning mechanism 18. Such a laser can be designed
to deliver PQSL pump light 20 in the form of a continuous wave (cw)
light beam. Many types of lasers are suitable for use as the PQSL
pump source 16. For example, diode lasers emitting PQSL pump light
20 within the 750 nm to 1100 nm range can be used. The power level
of these diode lasers can be between 100 mW and 4000 mW.
[0032] A lens 22 is provided before PQSL pump source 16 for
focusing pump light 20 and directing it to an input coupler 24 of
Q-switched laser 12. Input coupler 24 is designed to admit pump
light 20 into a cavity 26 of passively Q-switched laser 12. Cavity
26 has an optical path length L defined between input coupler 24
and an output coupler 28. Although in the present embodiment cavity
26 is linear and couplers 24, 28 are in the form of mirrors, a
person skilled in the art will appreciate that other types of
cavities and coupling elements can be used, see e.g.,
commonly-assigned U.S. patent application Ser. No. ______ (Agents
Docket Number LEL-010), which has been incorporated herein by
reference.
[0033] Cavity 26 contains a gain medium 30. Gain medium 30 exhibits
a high amount of gain per unit length when pumped with PQSL pump
light 20. Typically, high gain is achieved by providing a high
doping level in gain medium 30 within the cross section traversed
by light 20. Doped materials with suitable amounts of gain to be
used as gain medium 30 include Yb:YAG at the 1030 nm and 980 nm
transitions, Nd:Vanadate at the 880 nm, 914 nm, and 1064 nm
transitions and Nd:YAG at the 946 nm and 1064 nm transitions. A
person skilled in the art will be familiar with other suitable
materials and the corresponding transitions. Some of these
materials include Yb Glass Fiber (980 nm transition), Yb Glass
Fiber (1020-1120 nm transition), Nd Glass Fiber (880-940 nm
transition), and Nd Glass Fiber (1050-1090 nm transition).
[0034] Cavity 26 also contains a passive variable loss element or
passive Q-switch 32. Preferably, passive Q-switch 32 is a saturable
absorber Q-switch such as chromium:YAG, which functions in the
wavelength range from 860 nm to 1100 nm. Alternatively,
semiconductors or semiconductor material structured to act as a
mirror can be used as passive Q-switch 32. Passive Q-switch 32 is
adjusted for switching on and off such that, when subjected to cw
pumping by PQSL pump light 20, passively Q-switched laser 12
generates a pulsed primary beam 34 at a primary wavelength
.lambda..sub.p. For clarity, only a single primary pulse 36 of
primary beam 34 exiting cavity 26 through output coupler 28 is
indicated in FIG. 1. Primary wavelength .lambda..sub.p corresponds
to the selected transition of gain medium 30. This transition can
be selected in any suitable range. By way of example, the
transitions are selected in a wavelength range between 860 nm and
1100 nm.
[0035] The fiber amplifier system 10 also has a fiber pump source
38 for supplying a fiber pump light 40 to the fiber amplifier 14.
The fiber pump source 38 can be a diode laser operating in the
wavelength range from 750 to 1000 nm and delivering between 1 and
100 Watts of power. Preferably, fiber pump source 38 is fiber
coupled laser such as a LIMO type laser (available from LIMO Laser
Systems, of Dortmund, Germany). A lens 42 and a beam combiner 44
are positioned in the path of the fiber pump light 40. Lens 42
focuses the fiber pump light 40 such that it is in-coupled into the
fiber amplifier 14. In particular, with the aid of lens 42 the
fiber pump light 40 is in-coupled into a cladding 46 of the fiber
amplifier 14. A lens 48 is also positioned in the path of primary
beam 34 before beam combiner 44. Lens 48 focuses primary beam 34
such that after being combined with the fiber pump light 40 by beam
combiner 44, primary beam 34 is in-coupled into a core 50 of the
fiber amplifier 14.
[0036] The fiber amplifier 14 produces a pulsed intermediate beam
52 at primary wavelength .lambda..sub.p from primary beam 34.
Preferably, pulsed intermediate beam 52 exhibits high peak power,
e.g., in the range of a few thousand Watts in each pulse 54 (only
one pulse shown for reasons of clarity). To achieve such high peak
power fiber amplifier 14 has a short length D, e.g., D is on the
order of a few meters, so as to suppress stimulated Raman
scattering (SRS). One working example of a system like system 10
had a fiber with a length D of about 5 meters and a peak pulse
power of about 4 kW. In addition, to achieve efficient absorption
of the fiber pump light 40 in core 50 over such short length D,
cladding 46 is preferably small, e.g., between 100 .mu.m and 200
.mu.m in diameter or smaller. For example, air-clad fibers can have
pump claddings about 40 .mu.m in diameter. Furthermore, core 50 is
preferably large, e.g., between 5 .mu.m and 10 .mu.m mean diameter,
and exhibits a high doping level, e.g., 0.5% or more. A person
skilled in the art will be able to select the appropriate dopant
for doping core 50 to amplify primary beam 34 based on primary
wavelength .lambda..sub.p. Suitable doping ions when primary
wavelength .lambda..sub.p is in the green range are Ytterbium ions
while Neodymium ions can be used for amplifying primary beam 34
when its light is in the green or blue range.
[0037] A lens 56 and a beam guiding element 58, in this case a
mirror, are positioned in the path of pulsed intermediate beam 52.
Lens 56 shapes pulsed intermediate beam 52 and element 58 deflects
it such that beam 52 is in-coupled into the nonlinear element 60.
Nonlinear element 60 is selected for its ability to frequency
convert pulses 54 of pulsed intermediate beam 52 in a single pass
to produce a pulsed output beam 62 at an output wavelength
.lambda..sub.out. Only one pulse 64 of output beam 62 is
illustrated for clarity.
[0038] In the present embodiment, nonlinear element 60 consists of
a single nonlinear optical crystal capable of converting primary
wavelength .lambda..sub.p to output wavelength .lambda..sub.out in
the UV, green or blue range. The conversion process is second
harmonic generation (SHG) and is well-known in the art. SHG doubles
the frequency of intermediate beam 52, or, equivalently, halves
primary wavelength .lambda..sub.p such that
2.lambda..sub.out=.lambda..sub.p. Hence, when primary wavelength
.lambda..sub.p is in the range from 860 nm to 1100 nm output
wavelength .lambda..sub.out will be in the range from 430 nm to 550
nm.
[0039] Preferably, the optical crystal used as nonlinear element 60
is a borate crystal. More preferably, the optical crystal is a
lithium borate (LBO) or barium borate (BBO) crystal. Also, although
only one crystal is employed as nonlinear element 60 in the present
embodiment, several can be used, as will be appreciated by those
skilled in the art. In addition, any appropriate phase matching
technique known in the art is employed to ensure efficient SHG in
nonlinear element 60.
[0040] During operation, pump source 16 is tuned by mechanism 18 to
generate pump light 20 in the form of a cw beam at the requisite
wavelength to pump gain medium 30. Passively Q-switched laser 12 is
adjusted such that primary pulses 36 of primary beam 34 are
controlled. To achieve this, one notes that a round-trip time,
t.sub.rt, of cavity 26 is related to the optical path length L of
cavity 26 by the equation: 1 t n = 2 L c
[0041] where c is the speed of light. Hence, round-trip time
t.sub.rt can be set by selecting optical path length L of cavity
26. The optical path length L takes into account the indices of
refraction of the components that make up the cavity.
[0042] Meanwhile, passive Q-switch 32, (e.g., a saturable absorber
Q-switch) is adjusted by setting its inter-pulse time. This is done
by choosing the appropriate saturable loss, q.sub.o, of the
absorbing material and using the fact that the repetition rate of
passive Q-switch 32 is proportional to pump power or the power
level of pump light 20, and that increasing the repetition rate
produces longer primary pulses 36. These parameters can be adjusted
to obtain the appropriate inter-pulse time; for more information
see, e.g., G. J. Spuhler et al., "Experimentally Confirmed Design
Guidelines for Passively Q-Switched Microchip Lasers Using
Semiconductor Saturable Absorbers", J. Opt. Soc. Am. B, Vol. 16,
No. 3, March 1999, pp. 376-388 (hereinafter Spuhler), which is
incorporated herein by reference. Although Spuhler provides
adequate guidelines for PQSL systems providing 1064-nm output, PQSL
systems that produce 914-nm radiation, e.g., those using
Nd:YVO.sub.4 as the gain medium 30, present much greater problems.
Solutions to these problems are addressed in commonly-assigned,
co-pending U.S. patent application Ser. No. ______ (Agents Docket
Number LEL-0 00, which has been incorporated herein by
reference.
[0043] In a preferred embodiment, optical path length L is very
short, e.g., L is on the order of 10 millimeters or less.
Preferably, L is even less than 1 millimeter. The inter-pulse time
of passive Q-switch 32 is selected such that primary pulses 36 have
a pulse duration t.sub.p of about 100 times round-trip time
t.sub.rt as illustrated in FIG. 2. In addition, passive Q-switch 32
is also set such that the time between successive primary pulses 36
at times t.sub.i and t.sub.i+1 defining an interpulse separation is
at least 100 times pulse time t.sub.p and preferably up to 10,000
times pulse time t.sub.p. Thus, in the preferred embodiment,
primary pulses 36 have a duty cycle ranging from 0.01% to 1%.
[0044] Primary pulses 36 exiting passively Q-switched laser 12
should preferably have a peak power level of at least 10 Watts and
preferably between 50 and 500 Watts. When primary pulses 36 enter
fiber amplifier 14, which has a gain of about 100 or more (e.g.,
between 50 and 500) they are amplified to form intermediate pulses
54 with over 1,000 Watts and preferably over 10,000 Watts of peak
power while preserving primary pulse timing as described above. At
this power level and timing, intermediate pulses 54 have a pulse
format which is above a nominal nonlinear frequency conversion
threshold for SHG in nonlinear element 60. Specifically, for the
purposes of this description, nominal nonlinear frequency
conversion threshold is defined to correspond to a pulse conversion
efficiency of at least 10%. Preferably, the conversion efficiency
is close to 50% or even higher. Now, at 10,000 Watts of peak power
intermediate pulses 54 exhibit approximately 50% efficient
conversion to output pulses 64 in LBO or BBO crystals of 20 mm
length.
[0045] By operating fiber amplifier system 10 as described above it
is possible to obtain output beam 62 with output pulses 64 in the
wavelength range from 430 nm to 550 nm at up to 5,000 Watts of peak
power with a duty cycle between 0.01% and 1%. The actual
application for which the fiber amplifier system 10 is used will
determine the exact peak power requirements for output pulses 64
and the required output wavelength .lambda..sub.out.
[0046] The fiber amplifier system 10 is a compound source with a
number of elements requiring proper alignment and positioning.
Several components of the fiber amplifier system 10 can be
simplified to reduce the complexity and cost of the fiber amplifier
system 10. FIG. 3A illustrates a preferred embodiment of a
passively Q-switched laser 80 for the fiber amplifier system 10.
Laser 80 consists of a thin plate of saturable absorber 82 serving
as the passive Q-switch and of a thin plate of gain medium 84.
Saturable absorber 82 is bonded or otherwise attached to gain
medium 84. It is also possible to align the plates of saturable
absorber 82 and gain medium 84 in parallel and in close proximity.
In this event the facing surfaces of the plates should be coated
for low reflection.
[0047] A first mirror 86 and a second mirror 88 are deposited
directly on the external surfaces of the plates of saturable
absorber 82 and gain medium 84. First mirror 86 is an input coupler
and admits pump light 20 into laser 80. Second mirror 88 is an
output coupler, and serves for coupling out primary pulses 36 of
pulsed primary beam 34. Mirrors 86 and 88 define a resonant cavity
90 of length L, which is short, e.g., on the order of 1 mm or less.
Laser 80 is sometimes referred to as a microchip laser in the art.
For further information on design guidelines for microchip lasers
the reader is again referred to G. J. Spuhler et al.,
"Experimentally Confirmed Design Guidelines for Passively
Q-Switched Microchip Lasers Using Semiconductor Saturable
Absorbers", J. Opt. Soc. Am. B, Vol. 16, No. 3, March 1999, pp.
376-388. FIG. 3B illustrates another embodiment of a passively
Q-switched laser 100 for the fiber amplifier system 10. Laser 100
has a gain fiber 102 disposed in a resonant cavity 104. Resonant
cavity 104 is defined between a mirror 106 for in-coupling pump
light 20 and a mirror 108 for out-coupling pulsed primary beam 34.
Although cavity 104 is defined by mirrors 106, 108 in this case,
gratings or coatings placed near the end of gain fiber 102 could
also be used to define cavity 104. In fact, sometimes only one
grating or coating can be used and the other end of gain fiber 102
can be cleaved to obtain Fresnel reflection from the cleaved
surface. A person skilled in the art will appreciate how to process
gain fiber 102 to establish cavity 104.
[0048] Gain fiber 102 is doped with gain material, as is known in
the art. A saturable loss absorber 110 serving as passive Q-switch
is spliced with gain fiber 102. Alternatively, saturable loss
absorber 110 can be a segment of fiber doped with the saturable
absorber material or it can even be a separate segment of fiber
placed between the end of gain fiber 102 and mirror 108.
[0049] FIG. 4A illustrates in cross section a fiber amplifier 120
which can be used by the fiber amplifier system 10. Fiber amplifier
120 has an active, circular core 122 surrounded by a cladding 124
with an irregular cross section. Core 122 is preferably a
single-mode core. A protective outer cladding 126 surrounds
cladding 124. The fiber pump light 40 is in-coupled into cladding
124, while primary beam 34 is in-coupled into core 122, as
described above. Because of the irregular cross section of cladding
124, pump light 40 is efficiently delivered to core 122 for
amplifying primary beam 34. Thus, the length of fiber amplifier 120
can be kept short, e.g., 2 meters or less, as indicated above.
[0050] FIG. 4B illustrates yet another fiber amplifier 130 which
can be used by the fiber amplifier system 10. Fiber amplifier 130
has an active, circular core 132 surrounded by a first cladding
134. Cladding 134 has a circular cross section and is in turn
surrounded by a second cladding 136 with an irregular cross
section. Fiber amplifier 130 has a protective outer cladding 138.
The addition of cladding 134 and adjustment of its index of
refraction makes it possible for fiber amplifier 130 to alter the
propagation characteristics of fiber amplifier 130 to improve the
in-coupling of the fiber pump light 40 into core 132 and to improve
the amplification efficiency. Once again, this enables one to keep
the length of fiber amplifier 130 short.
[0051] A person skilled in the art will recognize that the
appropriate choice of fiber amplifier, its cross section, its
length as well as pulse time t.sub.p and pulse energy are required
to avoid fiber optic nonlinearities and especially those associated
with stimulated Raman scattering as well as stimulated Brillouin
scattering (SBS) and self phase modulation. However, to achieve
good efficiency in the nonlinear element 60, high powers must be
used. Although such high peak powers are good for the frequency
conversion in the nonlinear element 60, they are also bad for the
fiber amplifier 14. When high peak powers are put into the fiber
amplifier 14, nonlinear processes (e.g., Brillouin scattering,
Raman scattering, and self-phase modulation) degrade the
intermediate beam 52 and prevent the fiber amplifier 14 from
working as desired.
[0052] Therefore, it is critical to create a pulse train with the
right pulse width and repetition rate, so that the intermediate
beam 52 will be effectively frequency converted by the nonlinear
element 60, but not affected adversely by the nonlinear processes
that can occur in the fiber amplifier 14. The following discussion
addresses the optimization of the pulse width and repetition
rate.
[0053] II. Fiber Nonlinearities
[0054] A. Raman Scattering.
[0055] In a Raman scattering event, a photon is absorbed by the
silica of the fiber, and simultaneously another photon is emitted.
The emitted photon is shifted to the red by about 13.2 THz, or 440
cm.sup.-1, and it leaves behind a vibrational excitation in the
glass. The vibrational state then quickly dissipates into heat.
Raman scattering is an effect that has gain, so even though an
incident pulse at first generates only a few Raman photons, these
photons increase the rate of Raman scattering, until eventually the
entire pulse has been shifted to the red by the Raman effect. The
length of fiber required for this conversion to take place is the
Raman threshold length LR.
[0056] B. Brillouin Scattering.
[0057] Another effect that shifts the frequency of the light and
creates a vibrational excitation in the glass is Brillouin
scattering. Although it is quite similar to Raman scattering,
Brillouin scattering is considered separately for three reasons:
(1) the frequency shift is very small (about 10 Ghz); (2) the light
is scattered backward instead of forward; and (3) the gain profile
is extremely narrow (typically about 50 Mhz).
[0058] The Brillouin threshold length L.sub.B is the length of
fiber needed for the incident pulse to be converted to a
backward-traveling Brillouin wave. Assuming a
Fourier-transform-limited input pulse, the Brillouin threshold
length increases as the pulse duration decreases, since smaller
pulse times mean larger frequency spreads, and the narrow gain
bandwidth of Brillouin scattering requires a narrow input frequency
bandwidth for efficient scattering.
[0059] Since the incident pulse and the Brillouin scattered light
travel in opposite directions, the Brillouin light walks off from
the incident pulse quite rapidly. Even so, the length of fiber
required for the Brillouin wave to walk off, L.sub.BW, can easily
be longer than the Brillouin threshold length L.sub.B so that
Brillouin scattering will still reach threshold. The Brillouin
scattering limit to the power achievable in the fiber amplifier 14
can be determined by finding the power for which the Brillouin
threshold length L.sub.B is equal to the Brillouin walkoff length
L.sub.BW.
[0060] C. Self Phase Modulation.
[0061] The fiber amplifier 14 is typically made from a material
such as silica. The nonlinear index of refraction of silica and
other common fiber materials depends upon the intensity of light
present. Therefore, a primary pulse 36 passing through the fiber
amplifier 14 gains an extra phase that varies along the length of
the pulse according to the pulse's instantaneous intensity. The
rate of change of this phase is a frequency chirp that broadens the
frequency bandwidth of the pulse. The nonlinear length L.sub.N is
the length of fiber necessary for the peak of the pulse to gain an
extra phase of 2.pi.. This phase gives the pulse a frequency chirp
.delta..nu.. Typically, .delta..nu. should be less than about 600
Ghz, to ensure efficient frequency doubling by LBO.
[0062] Through an analysis of the combined effects of Raman
scattering, Brillouin scattering, and Self-Phase modulation, the
inventors have determined an optimum pulse format that is
compatible with both frequency conversion and pulse amplification.
The following section discusses this pulse format.
[0063] III. Pulse Format
[0064] A. Maximum Pulse Length
[0065] Since Brillouin scattering occurs in the backward direction,
the Brillouin wave quickly walks off from the initial pulse, and
therefore loses the source of its gain. So intuitively, shorter
pulses will be better, since the walkoff will occur faster, before
much power can be built up. Furthermore, as discussed above,
shorter pulses have a higher frequency spread, and this frequency
spread also increases the Brillouin threshold.
[0066] The situation is discussed by Govind Agrawal in his book
Nonlinear Fiber Optics (Third Edition, Academic Press, 2001).
Agrawal notes that Brillouin scattering occurs by building up an
acoustic wave. He introduces the phonon lifetime T.sub.B, which is
approximately equal to 10 nsec. Agrawal states that for "pulses of
width T.sub.0<T.sub.B, the Brillouin gain is substantially
reduced (p. 359)." Thus, based on Agrawal's analysis of Brillouin
scattering alone, one would expect that the threshold intensity for
Brillouin scattering to increase as the pulse length decreases.
But, in the system 10, the reason for decreasing the pulse length
of the primary pulses 36 is to increase the corresponding peak
intensity of these pulses. Thus it would appear that at least some
Brillouin scattering would be present no matter how short the
pulse.
[0067] However, Agrawal does not address the combined effect of
Brillouin scattering and self-phase modulation. As discussed above,
self-phase modulation broadens the spectrum of the amplified pulse.
This broadening further reduces the Brillouin scattering, and the
effect is greater for shorter pulses. In fact, the inventors have
discovered that there is a critical pulse length, shorter than
which Brillouin scattering will never reach threshold, because of
the self-phase modulation. As the pulse travels down the fiber, its
frequency spectrum broadens quickly enough that the Brillouin
scattering can never build up significant intensity. The inventors
have calculated the effect of self-phase modulation on the
threshold for Brillouin scattering. The results of these
calculations are shown in the graph of Brillouin threshold versus
pulse length of FIG. 9. From the graph it can be seen that below a
pulse length of about 1.7 nsec, the Brillouin threshold is, for
practical purposes, infinite. Thus, if the pulses from the source
10 have a pulse length of less than about 1.7 nsec, the Brillouin
scattering is not a problem.
[0068] B. Minimum Pulse Length
[0069] The shorter the primary pulses 36, the more their spectrum
broadens due to self phase modulation in the fiber amplifier 14. If
the spectrum of the primary pulses 36 is too broad, however, the
pulses 54 of the intermediate beam 52 often cannot be converted to
the desired wavelength by the nonlinear element 14, e.g., where the
nonlinear element is a frequency doubling crystal. Since the
frequency bandwidth of a pulse is inversely related to the pulse
length, the pulse length of the primary pulses 36 should be long
enough that frequency bandwidth of the corresponding intermediate
pulses 54 after they emerge from the fiber amplifier 14 is less
than the acceptance bandwidth of the nonlinear frequency converting
element 60. The lower limit on T.sub.pulse depends on the nonlinear
material used in the nonlinear frequency converting element. For
example, to maintain a usefully narrow frequency spectrum for
frequency doubling in lithium borate (LBO), the pulse width should
be greater than about 100 psec.
[0070] C. Minimum repetition rate
[0071] There are two reasons for wanting a high repetition rate.
First, for low repetition rates, amplified spontaneous emission
becomes a problem during the quiet times between pulses. During
these quiet times, the fiber is being charged up by the pump, with
no place for the energy to go. Eventually, spontaneous emission
will touch off a chain reaction that will extract the power from
the fiber, leaving nothing behind with which to amplify the PQSL
pulse. To prevent spontaneous emission in the fiber amplifier
between pulses from initiating such a chain, the repetition rate
needs to be sufficiently large that amplified spontaneous emission
in the fiber amplifier between pulses does not extract more than
50% of the total power from the fiber amplifier, e.g., greater than
about 1 00 kHz (typically). With respect to a PQSL such as the
fiber amplifier system 10 the repetition rate can be adjusted by
adjusting the power of the pump light 20. Generally speaking, the
greater the power of the pump light 20, the greater the repetition
rate of the PQSL.
[0072] Another reason that high repetition rates are desirable is
that light sources used in display systems often require a rapid
refresh rate. For example, in a grating light valve (GLV)-type
display, an entire column of pixels is illuminated at the same
time. If there are about 2000 columns in the image, and the image
changes at 50 frames per second, this requires a minimum of 100 kHz
repetition rate for the laser. However, to eliminate speckle, and
alleviate the need for exactly timing the pulses with the GLV
scanner, it is preferable to have a repetition rate of about 10
times the theoretical minimum, or about 1 MHz.
[0073] It has been difficult to obtain laser pulses between 50 ps
and 2 ns at such high repetition rates. Q-switched lasers typically
provide pulses greater than about 5 ns long. Mode-locked lasers
generally provide pulses less than 50 ps long. In addition,
mode-locked lasers tend to be much larger than Q-switched lasers.
For example, the largest dimension on a typical mode-locked laser
is typically on the order of one to two feet. The largest dimension
on a PQSL, by contrast, is on the order of one to two inches.
[0074] In order to make a PQSL with the desired pulse length, the
length L of the resonator cavity 26 is a critical parameter. There
are two reasons to make the resonator cavity 26 very short. First,
the pulses get shorter as the resonant cavity gets shorter. Second,
the PQSL will oscillate at a single frequency only if the resonator
is so short that it supports only one mode of oscillation. The
length of the resonator cavity such as that shown in FIG. 3A is
almost totally determined by the thickness of the gain medium 84.
However, if the gain medium becomes too thin, it won't absorb
enough of the PQSL pump light 20 to provide a useful intensity in
the pulsed primary beam 34. Usually it is desired to absorb as much
radiation as possible. However, the inventors have determined that
the PQSL 12 can operate effectively with the desired pulse length
even if the gain medium is so thin that it absorbs less than half
of the PQSL pump radiation. For 1064 nm, design of the PQSL to
obtain the desired pulse length is relatively straightforward.
Spuhler, e.g., indicates that the pulse period (pulse length)
T.sub.pulse for a PQSL can be determined from 2 T pulse = 3.52 t r
t q 0
[0075] where t.sub.rt is the round trip pulse time defined above
and q.sub.0 is the saturable loss in the passive Q-switch in the
PQSL. For radiation corresponding to certain transitions, e.g., the
914-nm transition in Nd, additional design considerations must be
taken into account. A PQSL for producing 914-nm is described in
U.S. patent application Ser. No. ______, (Agents Docket Number
LEL-010), which has been incorporated herein by reference.
[0076] IV. Fiber Design
[0077] In addition to the pulse format, optimized frequency
conversion requires optimization of the fiber amplifier 14. The
following discussion addresses issues of fiber design.
[0078] A. Figure of Merit
[0079] The inventors have found that, due to the limitations
imposed by Raman scattering, a fiber's capacity to generate light
depends on the product of its absorption .beta. of pump light 40
(measured in dB/m), and the mode area of the light to be amplified,
A.sub.mode measured in square microns (.mu.m.sup.2). The inventors
have derived for a fiber a "figure of merit", or "FOM" denoted by
the symbol z, which has a critical value that can be used to
optimize the fiber amplifier 14.
[0080] The figure of merit (FOM) z can be derived as follows. The
fiber amplifier 14 will produce an average output signal power S of
beam 52 from an average power P of pump light 40. Powers S and P
are related by an equation of the type:
S=.epsilon.P(1-e.sup.-.beta.'L.sup..sub.f)-.THETA.L.sub.f (1)
[0081] where .epsilon. is the conversion efficiency of the fiber
amplifier 14, L.sub.f is the fiber length in meters and .beta.' is
the fiber absorption coefficient for pump light in the pump guide
in units of (meters).sup.1. The fiber absorption .beta.' is defined
as: 3 ' = ln ( Q i Q t ) L f ;
[0082] where Q.sub.i is the amount of pump light coupled into the
pump guide of the fiber amplifier 14 and Q.sub.t is the amount of
pump light transmitted through the fiber amplifier 14. Thus,
.beta.' is a constant of the fiber used in the fiber amplifier 14
independent of the fiber length. As a practical matter the fiber
absorption can be determined as core absorption multiplied by the
ratio of the cross-sectional area of the core of the fiber to the
cross-sectional area of the core plus the pump guide. The term
.THETA.L.sub.f takes into account the possibility that the fiber
amplifier 14 may absorb radiation at the wavelength being
amplified, as is the case for 914-nm (but not 1064-nm) radiation in
Nd-doped fibers. The quantity .THETA. has units of Watts/meter and
represents a threshold signal power needed before absorption is
overcome and the fiber amplifier 14 can amplify. The derivation of
z proceeds by eliminating L.sub.f in favor of z. For fiber
amplifiers that do not significantly absorb the radiation being
amplified .THETA.=0, which is assumed for the rest of the
derivation for the sake of example. In practice, the resulting
figure of merit z does not change significantly even where
absorption of 914-nm cannot be neglected so .THETA.=0 can be
assumed for both 914-nm and 1064-nm radiation.
[0083] The Raman scattering threshold is determined by: 4 p L f g r
A mode = 16 ( 2 )
[0084] where p is the peak power of the input signal, g.sub.r is
the Raman gain and A.sub.mode is the cross-sectional area of the
amplified mode. A.sub.mode is defined as the area within which the
intensity of the amplified mode is not less than 1/e.sup.2 times
the intensity of the mode at its maximum. For a circular Gaussian
mode this is equal to .pi.d.sup.2/4, where d is the full width at
1/e.sup.2 times maximum. For an elliptical Gaussian mode A.sub.mode
is given by 9.pi.d.sub.majord.sub.minor/4, where d.sub.major and
d.sub.minor are the full width at 1/e.sup.2 times maximum along the
major and minor axes of the ellipse.
[0085] Solving equation (2) for L.sub.f, one obtains: 5 L f = 16 A
mode g r p . ( 3 )
[0086] When equation (3) is substituted back into equation (1) and
assuming .THETA.=0, one obtains: 6 S = P ( 1 - - 16 A mode g r p )
. ( 4 )
[0087] The form factor z can be defined as: 7 z = ' 16 g r A mode (
5 )
[0088] The value of the Raman gain g.sub.r is approximately
10.sup.-10 kW/m (see Agrawal, Nonlinear Fiber Optics, cited above).
Converting .beta.' in m.sup.-1 to .beta. in dB/m and expressing the
mode area in .mu.m.sup.2 one obtains:
z=(0.037).beta.(dB/m)A.sub.mode(.mu.m.sup.2) (6);
[0089] where 8 = ' 10 ln ( 10 ) 4.34 ' .
[0090] The nonlinear element 60 converts the amplified signal to
the output beam 62 at an efficiency .delta.(p) that depends on the
peak power p. The average power B(z, p) of the output beam 62
depends on the peak power p and the form factor z and is given by:
9 B ( z , p ) = ( p ) P ( 1 - - f z p ) ( 7 )
[0091] In equation (7), f denotes a correction factor that depends
on the pulse shape for the primary pulses 36 and the pumping
configuration of the fiber amplifier 14. For example, if the fiber
amplifier 14 has a double-pass pump configuration f gets a factor
of 2. Because p is not constant along the length of the fiber
amplifier 14 but increases with distance along the fiber f gets
another factor of 2. One would therefore expect a value of f
greater than or equal to 4.
[0092] For a fixed value of the figure of merit z one can determine
the best value p.sub.0 of the peak power p by solving 10 B ( z , p
) p p o = 0 ( 8 )
[0093] By plugging the best value p0 into equation (7) one can
obtain a best value of B as a function of z. A method for
optimizing the fiber amplifier using the figure of merit z can be
summarized as follows.
[0094] First the conversion efficiency .delta.(p) of the nonlinear
frequency converting element 60 is determined as a function of a
peak power of an input signal coupled into the fiber amplifier.
This can be done by experimental measurement or, in some cases can
be provided by the supplier of the nonlinear element 60. Next,
using equation (7), the average power of output radiation B(z, p)
from the nonlinear frequency converting element 60 can be
calculated as a function of the peak power p and a figure of merit
z. A best value p.sub.0 of the peak power p is then determined by
solving 11 B ( z , p ) p | p 0 = 0.
[0095] Next, a best value of the average power of the output
radiation B.sub.best(z) is determined as a function of the figure
of merit z by substituting p.sub.0 into equation (7).
[0096] A desired value B.sub.d of the average power of output
radiation from the nonlinear frequency converting element 60 is
determined from requirements of an application for which the fiber
amplifier system 10 is to be used.
[0097] From the desired value B.sub.d and the calculated
B.sub.best(z) one can determine a minimum value z.sub.min of the
figure of merit for the fiber, e.g., using graphical, numerical, or
analytical methods. From z.sub.min one can select a fiber amplifier
characterized by values of .beta. and A.sub.mode such that for the
fiber amplifier z is greater than or equal to z.sub.min.
[0098] From an analysis of the best value of B the inventors have
determined that, if the nonlinear element 60 is lithium borate
(LBO), to obtain sufficient intensity for frequency conversion in
the nonlinear element 60, the fiber amplifier 14 should have z
greater than about 0.1, preferably greater than about 0.5. If the
nonlinear element 60 has a higher nonlinearity than LBO, the fiber
amplifier 14 can have a lower value of z.
[0099] As a numerical example, for LBO, the conversion efficiency
.delta.(p) is given approximately by 12 ( p ) = A 1 + ( B p ) n ; (
9 )
[0100] where A=0.43; B=2.0 kW; and n=1.2.
[0101] Equation (9) can be substituted into equation (7). For the
sake of example the following values are assumed: P=20 W;
.epsilon.=0.50 and f=7.4 (determined experimentally). From
equations (7) and (8) one can obtain an equation for B.sub.best(z).
FIG. 10 depicts a graph of B.sub.best(z) for the present example.
If the desired average output power B.sub.d must be above some
threshold, the corresponding value of the FOM z for the fiber
amplifier 14 can be found from the graph. For example, for B.sub.d
greater than 2 W, z must be greater than about 0.6.
[0102] B. Core Index Range
[0103] It would appear at first glance that a sufficiently high Nd
concentration would maximize the absorption .beta. in Eq. (1).
However, in solution doping processes commercially used to dope the
core of fiber amplifiers, higher Nd concentration requires higher
.DELTA.n.sub.core, which means a smaller mode area. Therefore, the
absorption .beta., which is proportional to the product of the core
area and the Nd concentration, is not increased by higher Nd
concentration. In this case, it is better to have a low Nd
concentration; the core then can have a small refractive index,
allowing the mode area, and hence the FOM, to be large. To maximize
the FOM, the inventors have empirically determined that
.DELTA.n.sub.core typically has to be between about 0.0025 and
about 0.006. If the core index is too low, the core does not guide
the light well.
[0104] C. Elliptical Core.
[0105] It is possible to make a fiber amplifier such as that shown
in FIG. 4A or FIG. 4B, having a core with an elliptical
cross-section. The inventors have determined that using a fiber
amplifier with an elliptical core increases the FOM by a factor of
2 or so. An elliptical core is also preferable because it maintains
the polarization of the amplified light, so that beam 52 is
polarized. This polarization is important because nonlinear element
60 requires a polarized input to operate efficiently.
[0106] D. W-fiber.
[0107] A W-fiber has an index profile with depressed index cladding
surrounding the core. When the primary wavelength .lambda..sub.p is
approximately 0.91 .mu.m and the core 50 of fiber amplifier 14 is
doped with Neodymium (Nd), it is preferable to use a W-fiber for
the fiber amplifier 14 to suppress unwanted gain at approximately
1.05 .mu.m. The principle behind the W-fiber is based on the
observation that a typical fiber core surrounded by a cladding
always has at least one bound mode at any wavelength. FIG. 11A
depicts a graph 300 of refractive index n versus radial distance r
from the center of a typical optical fiber. The core region 302
typically has a higher refractive index than the cladding region
304. Total internal reflection takes place at the interface 306
between the core region 302 and the cladding region 304. However,
if a narrow region of lower refractive index than the cladding
region 304 surrounds the core region 302 light may tunnel out of
the core region 302. It is possible in such a situation that light
at certain wavelengths will have no bound modes. The situation is
depicted schematically in the graph 310 of FIG. 11B. In FIG. 11B, a
fiber has a core region 312 surrounded by a tunnel cladding region
313. A cladding region 314 surrounds the tunnel cladding 313 and
core region 312. The core is characterized by a refractive index
n.sub.core and a radius r.sub.c. The cladding region 314 is
characterized by a refractive index n.sub.cl and a thickness
t.sub.cl. The tunnel cladding region 313 is characterized by a
refractive index n' and a thickness t'. Generally,
n'<n.sub.cl<n.sub.core. Such a refractive index profile is
sometimes referred to as a "W" profile. The propagation of
radiation in fibers having such profiles is described in detail by
Michael Monerie in "Propagation in Doubly Clad Single-Mode Fibers",
IEEE Journal of Quantum Electronics QE-18 (1982) p. 525, which is
incorporated herein by reference, and references therein. If the
values of n.sub.core, n.sub.cl, n', r.sub.c, t.sub.cl and t' are
chosen such that an average squared index of refraction
(n.sup.2(r))><n.sub.cl.sup.2, then there exists a cutoff
wavelength .lambda..sub.c for which light having wavelengths (in
vacuum) greater than .lambda..sub.c will have no bound modes.
Undesired wavelengths above .lambda..sub.c will be scattered out of
the fiber along its length while bound modes of desirable
wavelengths below .lambda..sub.c are retained in the fiber. The
wavelength .lambda..sub.c is the cutoff wavelength of the
fundamental (LP.sub.01) mode. Generally the tunnel cladding region
313 is thick enough that (n.sup.2(r)><n.sub.cl.sup.2 but thin
enough to provide efficient tunneling of the undesired wavelengths.
Thus, by properly engineering the refractive index profile of a
fiber, it is possible to have a "W" profile wherein 0.91
.mu.m<.lambda..sub.c<0.05 .mu.m. For such a fiber, light of
wavelength 1.05 .mu.m will not have a bound mode and will pass out
of the fiber along its length. Light of wavelength 0.91 .mu.m will
have a bound mode that will be contained by the fiber. The overall
effect is to reject the undesired 1.05 .mu.m radiation while
retaining the desired 0.91 .mu.m radiation.
[0108] A specific embodiment of a practical application of this
principle utilizes a triply clad fiber illustrated by the
refractive index profile 320 of FIG. 11C. The fiber generally
comprises, as shown in FIG. 11C, a core region 322 surrounded by a
tunnel cladding region 323. A pump cladding region 324 surrounds
the core 322 and tunnel cladding 323 regions. An outer cladding
region 326 surrounds the core 322, tunnel cladding 323, and pump
cladding 324 regions. The core is characterized by a refractive
index n.sub.core and a radius r.sub.c. The tunnel cladding region
323 is characterized by a refractive index n', a thickness t' and
corresponding radius r.sub.tc=r.sub.c+t'. The pump cladding region
324 is characterized by a refractive index n.sub.pc and a thickness
t.sub.pc. The outer cladding is characterized by an index of
refraction n.sub.oc and a thickness t.sub.oc. The outer cladding
may be surrounded by air having an index of refraction of about
1.0. Generally, n'<n.sub.pc<n.sub.core and
n.sub.oc<n.sub.pc. Such a configuration allows the undesired
radiation to tunnel out of the core region 322. Total internal
reflection at an interface 325 between the pump cladding 324 and
outer cladding 326 provides a bound mode that confines the pumping
radiation for efficient pumping of the core region 322. Here,
<n.sup.2(r)> is defined as: 13 n 2 ( r ) = 1 A 0 r pc r r n 2
( r )
[0109] where r.sub.pc represents some distance from the axis of the
fiber that lies within the pump cladding and A represents a cross
sectional area of the fiber within r.sub.pc of the axis. For
example, if the fiber has a circular cross section,
A=.pi.r.sub.pc.sup.2. The radius r.sub.pc is typically greater than
a few undesired wavelengths.
[0110] It is also advantageous to use a W-fiber even when a
fundamental cutoff wavelength is not needed. This is because a
W-shaped index profile, such as that shown in FIG. 11B allows a
larger single-mode core than the single-step index profile depicted
in FIG. 11A. In particular, a single mode core is characterized by
a cutoff V-number V.sub.cl, defined as 14 V c1 = 2 r c c1 n core 2
- n c1 2
[0111] where .lambda..sub.cl is the second mode or LP.sub.11,
cutoff wavelength. For the single-mode core of FIG. 11A,
V.sub.cl=2.405. However, for the W-fiber of FIG. 11B, the
parameters r.sub.c, t', n.sub.core, and n' can be adjusted so that
V.sub.cl is 3.0 or greater. As such, the mode area and,
consequently, the FOM can be greater for a W-fiber than for a
single-step fiber.
[0112] Through appropriate use of a W-fiber, the fundamental cutoff
wavelength .lambda..sub.c of the fiber amplifier 14 can be
engineered to be above the primary wavelength .lambda..sub.p so
that the fiber amplifier system 10 can be used with to
preferentially generate blue or green light. A general discussion
of the selection rules for design of a fiber amplifier with a
particular cut-off wavelength .lambda..sub.c is described in detail
in commonly assigned U.S. Pat. No. 6,563,995, which is incorporated
herein by reference. Two examples, among others, of the use of such
a W-fiber for generation of blue light are as follows.
EXAMPLE 1
Neodymium Doped Fiber Amplifier
[0113] The amplifier includes the W-fiber having a core having
index n.sub.core, a depressed cladding having index n.sub.tc, and a
secondary cladding having index n.sub.pc, as described above. In
addition, the core is doped with Neodymium ions (on the order of
10.sup.20 ions per cm.sup.3, for example), and the secondary
cladding is surrounded by an outer cladding having a refractive
index n.sub.oc, where n.sub.oc<n.sub.pc. The secondary cladding
is used for guiding pump light that excites the Nd atoms. The
secondary cladding typically has a mean diameter between 40 .mu.m
and 80 .mu.m.
[0114] The secondary cladding is optically coupled to laser diodes
having a wavelength in the vicinity of 808 nm. The light from these
diodes creates gain in the core, both near 900 nm and near 1050 nm.
Light near 900 nm is input into the core and is to be amplified.
Light at 1050 nm is generated by the four level transition of Nd
atoms and is undesired. As an example, the light to be amplified
has a wavelength of 914 nm, corresponding to light from a laser
comprising a Neodymium-doped Yttrium vanadate crystal.
[0115] Thus, in this example, the indices of refraction n.sub.core,
n.sub.tc, and n.sub.pc and the radii r.sub.c and r.sub.tc are
selected to give a cutoff wavelength .lambda..sub.c between 914 nm
and 1050 nm. As an example, r.sub.c=3 .mu.m and r.sub.tc=6 .mu.m.
The secondary cladding is fused silica having n.sub.pc=1.458. The
outer cladding is a polymer cladding. The core is characterized by
n.sub.c-n.sub.pc=0.0022, and the depressed cladding has index
n.sub.tc given by n.sub.pc-n.sub.tc=0.0022. Accordingly, the fiber
in this example has a cutoff wavelength .lambda..sub.c of about 975
nm. The loss at 1050 nm is approximately 1400 dB/m.
EXAMPLE 2
Ytterbium Doped Fiber Amplifier
[0116] This example is similar to the Neodymium doped fiber
amplifier described in Example 1. The secondary cladding again
becomes a pump cladding. The core is doped with Ytterbium atoms.
When pumped with 920 nm light, the Ytterbium exhibits gain both at
980 nm and at approximately 1050 nm. The W-fiber parameters are
adjusted to give a cutoff wavelength .lambda..sub.c between 980 nm
and 1050 nm, with a suitable loss at 1050 nm.
[0117] E. Air Clad Fiber
[0118] The absorption .beta. (and hence the FOM) can be improved by
making the pump guide smaller. However, the numerical aperture (NA)
of the pump guide must simultaneously be made larger, so that light
can be coupled in. In a preferred embodiment, the fiber amplifier
14 uses a cladding-pumped fiber with a core surrounded by an air
cladding. Cladding pumped, air clad fibers are described, e.g., by
R. Selvas et al in "High-Power, Low-Noise, Yb-doped, cladding
pumped three-level fiber sources at 980 nm," Optics Letters, Vol.
28, No. 13, Jul. 1, 2003, which is incorporated herein by
reference. FIG. 12 depicts an example of air-clad fiber 400 having
an elliptical core 422. A depressed cladding 423 surrounds the core
422. A pump cladding 424 surrounds the depressed cladding 423. A
set of glass bridges 426 connects the pump cladding to an outer
cladding 428. Voids 425 between the glass bridges 426 provide an
air cladding that surrounds the pump cladding.
[0119] V. Alternative Fiber Amplifier System
[0120] FIG. 5 is a diagram of an alternative fiber amplifier system
140 according to another embodiment of the invention. In the fiber
amplifier system 140 a primary beam generator 142 combines a pump
source and a passively Q-switched laser and delivers a primary beam
144 having pulses 146 (only one indicated) of light at primary
wavelength .lambda..sub.p. The pulses 146 are formatted in
accordance with the guidelines given above.
[0121] The primary beam 144 is delivered to a fiber amplifier 148.
The fiber amplifier 148 amplifies the primary beam 144 to produce
an intermediate beam 150 still at primary wavelength
.lambda..sub.p. The intermediate beam 150 consists of pulses 152
(only one shown) which have a pulse duration, an inter-pulse
separation and peak power defining a format calibrated to obtain at
least 10% frequency conversion efficiency and preferably up to 50%
or higher frequency conversion efficiency in a nonlinear element
158.
[0122] A lens 154 and a beam guiding element 156 are placed in the
path of intermediate beam 150 for directing it to nonlinear element
158. Nonlinear element 158 has a waveguide 160 with a
quasi-phase-matching (QPM) grating 162 disposed therein. QPM
grating 162 is designed for phasematching the frequency conversion
operation by which intermediate beam 150 is converted to an output
beam 164 at output wavelength .lambda..sub.out. The frequency
conversion operation producing output beam 164 is second harmonic
generation (SHG). Conveniently, nonlinear element 158 with QPM
grating 162 is a PPLN, PPLT, PPKTP, MgO:LN or other poled
structure.
[0123] Alternatively, the frequency conversion operation can be
optical parametric generation (OPG) or another type of nonlinear
frequency conversion operation such as difference frequency
generation (DFG). OPG is an alternative to SHG because it is a
highly-efficient, single-pass and single input wavelength process
(the requisite idler and signal beams are usually obtained by
vacuum amplification). In addition, the output spectrum of output
beam 164 is somewhat broadened (typically by a few nm) when OPG is
used, making it more suitable for certain applications, e.g., for
image displays. On the other hand, when DFG is used as the
frequency conversion operation a beam 166 at wavelength .lambda.,
is required to mix with intermediate beam 150 in nonlinear element
158. In such situations pulses 168 (only one shown) of beam 166
should be synchronized with intermediate pulses 152. Also, beam
guiding element 156 is then adapted to function as a beam combiner.
Furthermore, a filter 170 can be provided for removing unwanted
frequencies exiting nonlinear element 158.
[0124] Several frequency conversion processes, i.e., a cascaded
nonlinear conversion process can be implemented in nonlinear
element 158 and use beam 150 in conjunction with beam 166 (and/or
other beams besides beam 166) or without it. Such operations may
involve several nonlinear operations in series. For example, second
harmonic generation followed by sum frequency generation, resulting
in third harmonic generation.
[0125] VI Image Projection System
[0126] In a particularly convenient embodiment of the invention
shown in FIG. 6 an image display system 200 employs a projection
light source 202. In this case image display system 200 is a
scanned linear image display system. Projection light source 202
has a first and a second light source (not shown in this figure) as
described above for producing output in the green wavelength range
and in the blue wavelength range, respectively. These two light
sources are used one after the other or sequentially for a certain
amount of time, as described below. Each of these two light sources
is set to deliver an output beam 206 at an average power of 2.5
Watts. For this purpose the duty cycle of the intermediate beam is
set at 0.05% and the peak power of intermediate pulses is set at
10,000 Watts. With this pulse format the conversion efficiency is
about 50%. Hence, output beam 206 will have an average power of 2.5
Watts (5,000 Watts of peak power at 0.05% duty cycle).
[0127] It is convenient to also provide projection light source 202
with a third light source producing output in the red wavelength
range. In this embodiment, the third light source is a diode laser
producing 2.5 Watts average power at a red wavelength. The output
of the third light source is coordinated with the output of the
first and second sources, such that only one color is present in
output beam 206 at a time.
[0128] Image projection system 200 has cylindrical beam shaping and
guiding optics 208, generally indicated by a cylindrical lens. Of
course, guiding optics 208 will typically include a number of
lenses and other elements, as will be appreciated by a person
skilled in the art. Optics 208 are adapted for line-wise image
scanning by expanding output beam 206 along the vertical direction.
An image generator 216 having a vertical line 218 of pixel
generators p.sub.i is positioned in the path of expanded output
beam 206. Image generator 216 can be any suitable unit capable of
generating images line-by-line and requiring illumination by red,
green and blue wavelengths in succession, as provided in output
beam 206. By way of example, image generator 216 may be a grating
light valve array made up of vertical line 218 of independently
controlled grating-type light valves 220. Each one of light valves
220 corresponds to a pixel generator p.sub.i. FIG. 7 illustrates a
light valve 220A having adjustable grating strips 222A. Strips 222A
are moved by a suitable mechanism to adjust the grating of light
valve 220A to diffract a particular color into a projection beam
228. The principles of operation and design of grating-type light
valves are known and the reader is referred for further information
to David T. Amm et al., "Optical Performance of the Grating Light
Valve Technology", presented at Photonics West--Electronic Imaging
1999, Projection Displays.
[0129] A linear scanner 210 having a rotating deflection unit 212
and a control 214 is provided for line-wise scanning of projection
beam 228. The scanning speed is controlled by control unit 214
which adjusts the angular speed of rotation .omega. of deflecting
unit 212. A person skilled in the art will recognize that other
types of optics and scanning devices can be used, depending on the
method of image scanning.
[0130] The scanned image produced by image generator 216 is
projected on a display screen 224 with the aid of optics 226,
generally indicated by a lens. In particular, light valves 220, are
set to diffract red, green and blue wavelengths provided in beam
206 to generate an image linewise in the diffracted projection beam
228. Beam 228 is projected by optics 226 on screen 224 to display
the image to a viewer. In one implementation certain light valves
220 are dedicated to each color. The image projected on the screen
224 is made up of a series of lines of pixels generated by the
pixel generators p.sub.i, e.g., 2000 lines. The pixel generators
p.sub.i sequentially generate lines of pixels as the as the
projection beam 228 scans horizontally across the screen 224.
[0131] Preferably, in this case valves 220 are subdivided into
groups of three one for diffracting blue, another for diffracting
green and a third one for diffracting red into projection beam 228.
Alternatively, light valves 220 can be modulated to diffract
different colors at different times (e.g., by
time-multiplexing).
[0132] A synchronizing mechanism 230 is connected to projection
light source 202 and to control 214 of linear scanner 210.
Mechanism 230 is provided to coordinate the timing of output pulses
232 in output beam 206 with the line scanning performed by linear
scanner 210.
[0133] When operating image display system 200 projection light
source 202 is set to deliver output pulses 232 at the green
wavelength from light source one, at the blue wavelength from light
source two, and at the red wavelength from light source three. The
pulses are repeated at a certain rate (i.e., at the inter-pulse
rate set as described above). Specifically, as better illustrated
in FIG. 8, light source 202 is set to deliver a number q of pulses
232 during a refresh time t.sub.refr which is the time allotted by
control 214 of linear scanner 210 to generating each line of the
image. Preferably, the number of pulses 232 during refresh time
t.sub.refr should be an integer multiple of the refresh rate, e.g.,
6 or more pulses 232 per refresh time t.sub.refr (i.e., q=6). For
better visualization, FIG. 8 illustrates the q pulses 232 delivered
by projection light source 202 during each refresh time
t.sub.refr.
[0134] The number q is dictated by the angular velocity .omega. of
rotating deflection unit 212. Synchronizing mechanism 230 adjusts
the timing of output pulses 232 in coordination with angular
velocity .omega. of unit 212 such that number q of pulses 232
delivered during each refresh time tree is equal. The refresh time
t.sub.refr is dictated by, among other things, the perception
parameters of the human eye. The light valves 220 in each pixel
generator p.sub.i in vertical line 218 have to respond sufficiently
fast that the resulting pixels in the scanned image can be
refreshed rapidly enough that the human eye does not perceive any
appreciable image discontinuities. This condition determines the
length of refresh time t.sub.refr given the number of lines of
which the scanned image is composed.
[0135] In display systems with a large number of lines, e.g., on
the order of 1,000 to 2,000 the appropriate refresh rate requires
that passively Q-switched laser for the first and second light
sources (green and blue) be set at a primary pulse repetition rate
of at least 100 kHz.
[0136] The light source of the invention can also be used in image
displays which are not scanned line-by-line but employ some
different scanning procedure. It can also be used in display
systems using as image generating pixels liquid crystals or
micro-mirror arrays. In still another embodiment, the light source
of invention can be used to illuminate a two-dimensional array of
pixels generating an image in a non-scanned image display system. A
person skilled in the art will appreciate that various multiplexing
and scanning methods can be employed to operate such scanned and
non-scanned display systems. Additionally, a person skilled in the
art will recognize that the applications of the light source in a
display system is only one of the many applications for this light
source can be used.
[0137] While the above is a complete description of the preferred
embodiment of the present invention, it is possible to use various
alternatives, modifications and equivalents. Therefore, the scope
of the present invention should be determined not with reference to
the above description but should, instead, be determined with
reference to the appended claims, along with their full scope of
equivalents. The appended claims are not to be interpreted as
including means-plus-function limitations, unless such a limitation
is explicitly recited in a given claim using the phrase "means
for."
* * * * *